It is known that a nuclear fuel for pressurized-water reactors may comprise cladding tubes built up of two layers. EP 301 295 describes nuclear fuel elements with cladding tubes with an inner supporting part of zirconium with 1.2 to 2% Sn, 0.07 to 0.2% Fe, 0.05 to 0.15% Cr, 0.03 to 0.08% Ni and 0.07 to 0.15% O or zirconium with 1.2 to 2.0% Sn, 0.18 to 0.24% Fe, 0.07 to 0.13% Cr and 0.10 to 0.16% O. The outer layer shall be of a zirconium alloy with better corrosion resistance than the inner part of the tube and is described to comprise either one or more of the alloying additives Sn, Fe, Cr, or Ni in a total amount being less than 1%, or one or more of the alloying additives Sn, Fe, Cr or Ni in a total amount less than 1% and 0.2 to 3% Nb. The described examples of outer layers with the above composition and improved corrosion properties are zirconium with 2.5 Nb and zirconium with 0.25% Sn, 0.5% Fe and 0.05% Cr.
A problem which arises with a cladding according to EP 301 295 is due to the great difference in Sn content which exists between the inner part, which has 1.2 to 2% Sn, and the outer layer with 0.25% Sn. Since the Sn content influences the recrystallization temperature of the zirconium alloy, the cladding will after the final heat treatment, which for pressurized-water fuel usually is a stress-relieve anneal, have an inner part which is stress-relieve-annealed and an outer part which is recrystallization-annealed. Such a difference in state between the inner part and the outer layer leads to different hydride orientation in these parts and to accumulation of hydrides at the boundary layer between the parts. Hydrides in the cladding material occur due to hydrogen developed during the corrosion of zirconium during reactor operation being taken up to a certain part by the cladding material, and since zirconium has low solubility of hydrogen, the absorbed hydrogen will be precipitated in the form of zirconium hydrides. These are elongated in shape and very brittle.
During the manufacture of cladding tubes for nuclear fuel elements, it is therefore important to see to it that hydrides which are precipitated in the cladding material are evenly spread in the material and tangentially distributed in the cross section. The hydrides are very brittle and may act as indications of fracture. It is therefore important that the hydrides are precipitated tangentially in the cross section of the tube and that there are very few radially directed hydrides which may act as indications of a crack through the cladding wall.
Nuclear fuel with a cladding consisting of two layers is also known from WO 93/18520. This describes a two-layer cladding, where both layers contain the same alloying elements to facilitate the handling of material returned from the manufacturing process, and whereof these are at least Sn, Fe and Cr. The supporting inner layer comprises 1-2% Sn, 0.05-0.25% Fe, 0.05-0.2% Cr to the outer layer 0.5-1.3% Sn, 0.15-0.5% Fe and 0.05-0.4% Cr and to obtain improved workability and bonding between the layers during the manufacturing of the cladding, the ratio between the contents of Sn in the outer layer and the inner layer shall be within the interval 0.35 to 0.7 and the content of Sn in the inner layer shall be two to five times the content of Fe and Cr in the outer layer.
The problem with this kind of nuclear fuel is that also for these layers, there may be great differences in the Sn content between the outer and inner layers. The outer layer in WO 93/18520 is stated to contain, besides Sn, Fe and Cr and then preferably 0.28.+-.0.04% Fe and 0.17.+-.0.03% Cr in order not to obtain an alloy which is difficult to work. An example given is zirconium with 1.1% Sn, 0.4% Fe and 0.25% Cr.
Other known fuel elements with a two-layer cladding are described in EP 212 351 which shows an outer layer consisting of zirconium with additives of Fe, V, Pt or Cu. EP 380 381 describes outer layers of zirconium with 0.35 to 0.65% Sn, 0.2 to 0.65% Fe, 0.24 to 0.35% Nb.